The invention lies in the field of processes for the recovery of aluminum from its ores using crystallization procedures.
The present invention is a combination of the process for producing phosphoric acid from apatite rock by the wet process and the production of aluminum from alunite in accordance with the basic process, the process resulting in the production of aluminum and monobasic potassium phosphate as the products. In the operation of the process, acid produced from apatite rock is cycled directly to the basic process where it is used with by-product potassium sulfate to produce monobasic potassium phosphate by ion exchange.
The basic process is defined by the following process steps, some of which are optional, as illustrated in the flow diagram of the referenced patent applications:
1. ROASTING PARTICULATE ALUNITE CONCENTRATE TO REMOVE WATER OF HYDRATION;
2. REMOVING SULFUR AND ALKALI METAL COMPOUNDS FROM THE ROASTED ORE BY A PROCEDURE INCLUDING A FINAL LEACHING STEP RESULTING IN A RESIDUE, AND A SOLUTION CONTAINING POTASSIUM SULFATE AS A BY-PRODUCT.
3. DIGESTING THE RESIDUE FROM THE LEACHING STEP WITH CAUSTIC SOLUTION TO CONVERT ALUMINUM VALUES IN THE RESIDUE TO SOLUBLE ALUMINATE;
4. PRECIPITATING SILICA FROM THE RESULTING SOLUTION OF STEP (3) TO LEAVE A SOLUTION OF ALUMINATE AND A RESIDUE KNOWN AS A DESILICATION PRODUCT CONSISTING ESSENTIALLY OF SODIUM ALUMINUM SILICATE ALONG WITH OTHER IMPURITIES;
5. PRECIPITATING ALUMINUM VALUES FROM THE SOLUTION OF STEP (4); AND
6. RECOVERING PRODUCT ALUMINUM FROM THE ALUMINUM VALUES OF STEP (5).
The recited process includes the alternative of recovering by-product potassium sulfate by crystallization as disclosed in U.S. Ser. 453,225 in which some potassium sulfate is bled off from the crystallization step. This potassium sulfate bleed stream is included in the term "by-product potassium sulfate" from the basic process referred to herein. Another alternative in the basic process is circulation of the mother liquor from step (5) which contains sodium hydroxide to the digestion step with some of the mother liquor being bled off from the precipitation step as impurities build up in it. In step (2) of the above-recited process, the sulfur and alkali metal compounds may be removed either by the procedure of U.S. Ser. 453,234 in which the roasted ore is subjected to a reducing roast and then to an oxidizing roast followed by leaching with a solvent, such as water or an alkali metal hydroxide, or by the process of U.S. Ser. 453,225 in which the roasted ore is leached directly, with a solvent, such as with ammonia or alkali metal hydroxide. Both of these step (2) procedures are included in the expression used herein, "roasting and leaching the alunite ore to remove water and compounds of sulfur and alkali metals."
Processes for the production of aluminum and monobasic potassium phosphate must be as economical as possible in order to produce competitive commercial grade products. This necessarily means conducting the production processes with a minimum of process steps and the recovery and utilization of as many by-products as possible. The combination of processes for the production of two or more products with the elimination of processing steps and the recovery and reuse in the combined process of materials which are normally by-products in the single processes is a distinct advantage. This is particularly true in this instance in which the organization producing the products by different processes is the owner ore bodies from which the two products are produced by disconnected processes.
In the production of phosphoric acid from apatite rock by the wet process for sale as such or for use in the manufacture of fertilizer, a high degree of purity of the product is required to make it commercial grade. Concentration and other purification steps are necessary as it cannot be marketed in the impure form in which it is produced. The purification procedure is a substantial item of expense in the production of a saleable product. If the raw phosphoric acid could be used without purification to make a commercial grade product, such as fertilizer, which is saleable with a minimum of purification, it would result in a realization of a higher profit for the phosphoric acid itself due to the elimination of the purification steps. This is especially true when the product is monobasic potassium phosphate because its value and marketability is much greater than that of phosphoric acid and potassium sulfate combined.
Likewise, the recovery of potassium sulfate from potassium sulfate solution resulting from the first leaching step of the basic process normally requires evaporation, crystallization, and other procedures which add to the cost of producing it for sale. Such procedures must be used in order to produce a commercial grade product. Accordingly, as is true with raw phosphoric acid produced by the wet process, if the impure potassium sulfate could be used to produce a commercial grade product, such as, monobasic potassium phosphate, the profit realized for the by-product potassium sulfate from the basic process would in the end be increased as the expensive purification procedures are eliminated.
In the elimination of silica by precipitation as sodium aluminum silicate from the aluminate produced in the digestion step of the basic process, the sodium aluminum silicate is normally discarded as a waste product. If some utility were found for this normally discarded product in the referred to combination of the two separate processes for producing phosphoric acid and aluminum, the economics for producing the end products would be further improved.
Accordingly, it is an object of this invention to provide a combined process for producing phosphoric acid from apatite rock and aluminum from alunite ore in which process steps are eliminated and by-products and normally discarded materials are recovered and reused in the combined process.